Geological faults occur when there is a fracture along which blocks of the Earth's crust (e.g., fault blocks) on either side of the fracture move relative to one another and parallel to the fracture (e.g., a fault plane). By definition, the fault block located above the fault plane is referred to as the hanging wall block and the fault block located below the fault plane is referred to as the footwall block. Different types of faults are classified based on the orientation of the fault blocks. For example, a “normal fault” occurs when the hanging wall block moves down relative to the footwall block and may occur when there is an expansion of the crust. Alternatively, a “reverse fault” occurs when the hanging wall block moves up relative to the footwall block and occurs when the crust is compressed.
Because faults in the Earth's crust cut through geological formations and offset them, the presence of faults makes property modeling of geological formations difficult.